butadiene + butadiene + + butadiene--→polybutadiene

(4 000 time)

丁二烯 ＋丁二烯＋ ＋丁二烯——→聚丁二烯

（4000次）

One can thus see how a substance (monomer) with as small a molecule weight

as 54 grow to become a giant molecule (polymer) of (54×4 000≈)200 000 molecular weight. It is essentially the ‘giantness’ of the size of the polymer molecule that makes its behavior different from that of a commonly known chemical compound such as benzene. Solid benzene, for instance, melts to become liquid benzene at

5.5℃ and , on further heating, boils into gaseous benzene. As against this well-defined behavior of a simple chemical compound, a polymer like polyethylene does not melt sharply at one particular temperature into clean liquid. Instead, it becomes increasingly softer and, ultimately, turns into a very viscous, tacky molten mass. Further heating of this hot, viscous, molten polymer does convert it into various gases but it is no longer polyethylene. (Fig. 1.1) .

因而能够看到分子量仅为54的小分子物质（单体）如何逐渐形成分子量为200000的

大分子（高聚物）。实质上，正是由于聚合物的巨大的分子尺寸才使其性能不同于象苯这样的一般化合物。例如，固态苯，在5.5℃熔融成液态苯，进一步加热，煮沸成气态苯。与这类简单化合物明确的行为相比，像聚乙烯这样的聚合物不能在某一特定的温度快速地熔融成纯净的液体。而聚合物变得越来越软，最终，变成十分粘稠的聚合物熔融体。将这种热而粘稠的聚合物熔融体进一步加热，不会转变成各种气体，但它不再是聚乙烯（如图

1.1）。

固态苯——→液态苯——→气态苯

加热，5.5℃ 加热，80℃

固体聚乙烯——→熔化的聚乙烯——→各种分解产物-但不是聚乙烯

加热 加热

图1.1 低分子量化合物（苯）和聚合物（聚乙烯）受热后的不同行为

Another striking difference with respect to the behavior of a polymer and that

of a low molecular weight compound concerns the dissolution process. Let us take,